The present invention relates to a process according to the preamble of claim 1 for preparing synthetically produced pigment particles containing, in particular, calcium carbonate, whereby foreign substances are added to the particles in connection with the forming of said particles in order to modify their properties.
According to a process of the present kind, a starting material containing calcium oxide is reacted with carbonate ions and other modification chemicals in the presence of water. Alternatively, the other modification chemicals are left unreacted among the rest of the material. The starting material may also comprise dry slaked Ca(OH)2 either together with unslaked lime or mixed therewith.
The use of calcium carbonate, particularly precipitated calcium carbonate, is becoming increasingly common in many fields of industry, such as within the paper, the plastics and the pharmaceuticals industry. The aim is to formulate precipitated calcium carbonate (PCC) into a finely divided, pure pigment, the optical properties of which, e.g. the brightness, are important properties for many applications. Synthetic calcium carbonate (SCC) is a generic term covering also other preparation processes than traditional precipitation in liquid phase.
There are several known methods for preparing PCC. In our earlier FI Patent Application No. 950411 it is mentioned that very finely divided PCC pigment can be prepared by using finely divided slaked lime as a starting material and by allowing the crystals grow essentially without mixing and by interrupting the reaction after a specific particle size has been reached by vigorous agitation.
In FI Patent Application No. 964132, said method has been further developed by providing for monitoring of the viscosity of the nucleation mass in order to find out the proper point of time for interrupting the growth of the particles.
FI Patent Application No. 971161 discloses carbonation of calcium hydroxide with carbon dioxide in a mixing apparatus having high energy density, said energy intensity being greater than 1000 kW/m3 in the free space of the mixing zone of the apparatus.
Precipitation of calcium carbonate on the surface of foreign particles is described in, e.g., published EP Patent Applications Nos. 0 375 683 and 0 604 095.
By the above-mentioned processes, a PCC product in slurry form is obtained which has to be filtered if a completely dry product is to be recovered.
The present invention aims at providing a technical solution for directly producing PCC in powder form without the need of first having to separate the product from a slurry, e.g. by filtration. The invention also aims at providing a process for easily modifying the properties of the PCC product in connection with the preparation process by combining desired modification chemicals therewith.
We have carried out tests aiming at preparing calcium carbonate which is as finely divided as possible. Surprisingly we have found in connection with these tests that the amount of water needed for ion formation during the intermediate stage of the synthesis is very small in comparison to prior experiences and knowledge. Without restricting ourselves to any particular theory, it appears to us that the phenomenon can be explained by the fact that even if a small amount of water is capable of dissolving only an infinitely small amount of soluble ions needed for formation of said ions, the extremely large mass transfer rate of the present process compensates for the water amount needed for normal precipitation.
The present invention is based on the concept that successive processes, such as slaking of lime, i.e. calcium oxide, and carbonation of the slaked lime, are carried out in a high-energy apparatus in which turbulence provided by the high energy intensity in the apparatus replaces a slow process based on diffusion only in liquid and gas. The reactions of the process are carried out at maximum dry matter content, in powder form, and as a result, the end product does not have to be concentrated e.g. by filtering or by other methods but the end product is useful as such e.g. for the production of a slurry which can be employed as filler or coating material of paper. Generally it can be noted that there is at least 20 parts by volume of gas in the carbonation reaction for each part by volume of a suspension formed by water and solid substances [primarily CaO, Ca(OH)2 and CaCO3)]. In practice, the water demand then only corresponds to the amount evaporated during the reaction (under the influence of the exothermal reaction and/or the processing temperature) together with the amount left in the Ca carbonate product which behaves as a powder. Thus, when the temperature of the gas is, for example, about 100xc2x0 C., only about 0.8 to 1.2 parts by weight of water are needed for each part by weight of the Ca starting material. There will be left a maximum of about 40% of water in the product. In the process according to the invention, water (process water) is used as reaction water and for heat transfer/cooling.
According to the present invention calcium carbonate is not precipitated on the surface of foreign particles in a continuous phase, as disclosed in published EP Patent Applications Nos. 0 375 683 and 0 604 095. Nor do earlier publications on PCC preparation suggest that carbonation could be initiated before the calcium oxide is slaked. According to the present invention the formation of the calcium carbonate takes place directly from calcium oxide or calcium hydroxide without intermediate stages in the form of a heterogeneous, three-phase synthesis. In the present context, xe2x80x9cthree-phase synthesisxe2x80x9d means that during the formation of the calcium carbonate there is present a solid phase (calcium oxid/calcium hydroxide/calcium carbonate), a liquid phase (water and optionally modifying agents dissolved in water) and a gas phase (carbon dioxide). The calcium carbonate is formed in the liquid phase present on the surface of a solid phase, and the calcium carbonate is released from the solid phase which forms its substrate of generation. The continued growth of the released calcium carbonate crystals is stopped because they are not in contact with the reactant substrate longer. The release of the particles takes place under the influence of three different features: The strong growth of the solid phase during the reaction; the strong temperature increase caused by the generation of reaction heat; and further the extremely strong turbulence in the apparatus.
Hydration of calcium oxide and carbonation of the hydratated part are performed one immediately after the other under the influence of efficient mixing. Then, according to the present invention, extremely small particles are at once formed which then immediately coalesce to 20 nm primary particles which agglomerate to form strong 50 nm aggregates which further generate loose 100 nm secundary aggregates which correspond to the balance between the forces acting on the particles.
These forces are, e.g., the capillary force caused by the water content, the van der Waals force, the mechanical forces caused by the turbulence of the mixing and the electric forces caused by the Z-potential. Because the process by itself gives rise to a pH in the excellent range of about 11, the isoelectric point of the Z-potential is close and there are no great resistance to the van der Waals forces. According to our calculations and measurements, a 1 to 5 molecules thick layer of water is formed on the surfaces of the particles. All ion reactions and non-ionic precipitations take place via said layer.
More specifically, the solution according to the present invention is mainly characterized by what is stated in the characterizing part of claim 1.
The present invention provides considerable advantages. Thus, the once-through time of the process from raw materials to end product is only on the order of some seconds. The present invention gives rise to a multifunctional process in which operations of earlier solutions are combined to produce the desired end product with extremely short residence time and with a small operational content. Simultaneously, it has become possible to remove intermediate depots and it has furthermore been found the the high-intensity mixing works best when the dry matter content of the treated materials is high with respect to water.
The behaviour of water in the process has been surprising. It is known that water in the form of steam is not capable of solubilizing salts, but we have learned that water spread out over a large surface in the form of a 1 to 5 molecules thick layer under the influence of the surface properties (in the present case of the SCC), does not behave as a solvent, either. The phenomenon surprisingly provides an option of influencing the crystal structure in a new way. This takes place when a suitable amount of water has been transferred to the gas phase by evaporation under the influence of a temperature increase.
The size of the forming particles can be regulated by adjusting the pH range, e.g. with NaOH or H2SO4 and by changing the intensity of the mixing and/or adjusting the initial amount of water. The starting materials of the process according to the invention comprise water, CaO/Ca(OH)2 and CO2. Of these substances, only calcium oxide and calcium hydroxide are actual variables. According to the invention it is preferred to have the CaO ready slaked or to slake it in the process by means of so called dry slaking, under vigorous agitation so that the Ca(OH)2 structure become porous and the size of the formed particles is  less than 3 microns and preferably  less than 1 microns, when the agitation during the slaking is sufficiently intensive.
A particle produced by the above-defined three-phase heterogeneous synthesis is opaque (i.e. it does not give any particular direction to light) and its morphology is originally vaterite. This morphology is very suitable for coating of paper, because high opacity can be obtained.
The calcium carbonate produced by the invention lends itself to use in particular not only as a coating pigment of paper and cardboard but also as a filler of paper and cardboard. It can, however, also be employed as a filler and pigment for polymers, such as plastics and rubbers, and paints and similar dispersions. The powdery product can be mixed with water to form a mixture having a desired dry matter content of, for example, about 60 to 80%.